Polymeric photoconductors of n-vinylindole and their use in electrophotographic processes

ABSTRACT

POLYMERIC PHOTOCONDUCTORS OF N-VINYLINDOLE, BOTH UNSUBSTITUTED AND SUBSTITUTED, COPOLYMERIZED THROUGH ITS VINYL SUBSTITUENT WITH A COREPEATING UNIT HAVING THE STRUCTURAL FORMULA:   -C&lt;(-C(-)-R&#39;&#39;-)   WHEREIN R&#39;&#39; IS A FUSED RING RADICAL. EXAMPLES OF THESE COPOLYMERS ARE N-VINYLINDOLE-INDOLE COPOLYMER, N-VINYLINDOLE N-METHYLINDOLE COPOLYMER, N-VINYLINDOLE-INDENE COPOLYMER, N-VINYLLINDOLE-COUMARIN COPOLYMER, AND NVINYLINDOLE-MALEIC ANHYDRIDE COPOLYMER.

United States Patent Oflice Int. Cl. G03g U.S. Cl. 96-1 Claims ABSTRACT OF THE DISCLOSURE Polymeric photoconductors of N-vinylindole, both unsubstituted and substituted, copolymerized through its vinyl substituent with a corepeating unit having the structural formula:

wherein R is a fused ring radical. Examples of .these copolymers are N-vinylindole-indole copolymer, N-vinylindole-N-methylindole copolymer, N-vinylindole-indene copolymer, N-vinylindole-coumarin copolymeryand N- vinylindole-maleic anhydride copolymer.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a class of N-vinylindole copolymers and, more particularly, relates to their preparation and use as photoconductors in electrophotographic processes.

Description of prior art The polymerization of N-vinylindole with stannic chloride and with organic electron acceptors are described in Polymer Science USSR, 8, 1394 ((1966) and in Polymer Letters, 4, 6 23 (1966), respectively. However, both of these articles are limited to only the polymerization of N-vinylindole and describe low molecular weight polymers of this compound, i.e.600l800 molecular weight with a softening range of 169-177 C. in the former and 5400 molecular weight inthe latter. Moreover, there is no mention of the photoconductive property of this compound.

In addition, a number of organic photocondu'ctors are known in the prior art, but relatively few of these are polymeric photoconductors which have the advantage of being formed into a photoconductive layer without requiring a non-photoconductive binder.

SUMMARY OF THE INVENTION A class of N-vinylindole copolymers having the following structural formula:

L 1. t1... La .11.

wherein R is selected from the group consisting of hydrogen and lower alkyl, wherein R is a fused ring radical, and wherein n is at least 1 and m+n is greater than 10.

In general, the N-vinylindole monomers can be prepared according to the above-cited Polymer Letters references. Alternatively, N-vinylindole and N-Vinyl-Z-methylindole and 3-methylindole can be prepared in one step 3,554,741 Patented Jan. 12., 1971 by pressure vinylation at elevated temperatures under nitrogen in a solvent containing an alkaline catalyst. For example, the vinylindole monomers were prepared from the corresponding indoles by the following procedure: The indole, g., was dissolved with 5 g. KOH in 250 m1. toluene in a one-liter stainless steel pressure reactor. The reactor was pressurized at ambient temperature to 200 p.s.i.g., with acetylene, 99.5% and diluted with 200 p.s.i.g. dry nitrogen. The temperature was slowly raised to 150"for the 2-, B-methylindoles and for indole and held constant for twenty-four hours. The extent of reaction was monitored on a pressure-temperature recorder. The crude mixture was washed with water and ether, the organic layer dried over Na S0 and fractionated through a glass helix-packed column under vacuum to give the monomers: N-vinylindole, B.P. 702 at 1 mm.; N-vinyl-2-methylindole, B.P. 105 at 2 mm.; and N-vinyl-3-methylindole, B.P. 94 at 1 mm.

Further purification of the monomers was required to remove the traces of unreacted starting compounds shown by thin layer chromatography and infrared. The fractionated products were washed several times with cold alkaline methyl alcohol (pH 9), extracted with ether, dried over Na SO and stripped under vacuum. Thin layer chromatography confirmed the purity of the monomers showing only one spot for each compound. The infrared spectra showed no N-H absorption at 2.94 7.

Analysis.Calct1lated for N-vinylindole (C H N) (percent): C, 83.88; H, 6.34; N, 9.78. Found (percent): C, 83.43, 83.64; H, 6.38, 6.15; N, 9.71, 9.89.

Analysis.Calculated for N-vinyl 2 methylindole (C H N) (percent): C, 84.08; H, 7.00; N, 8.92. Found (percent): C, 84.13, 84.16; H, 7.05, 7.14; N, 8.72, 8.88.

Analysis.Calculated for N-vinyl 3 methylindole (C H N) (percent): C, 84.08; H, 7.00; N, 8.92. Found (percent): C, 83.97, 84.09; H, 7.15, 7.15; N, 8.73, 8.73.

Indene, coumarin, maleic anhydride, benzofuran, and benzothiophene are commercially available.

In general, the polymerization of the N-vinylindole, substituted and unsubstituted, and copolymers is carried out in solution under an inert atmosphere with a cationic catalyst, such as diethylaluminum chloride. Alternatively, the copolymers are also prepared in vacuo with heat. In preparing these copolymers, a 1:1 molar ratio of each monomer is preferred in order to obtain a 50/50 copolymer. If desired, howev'e'f, other ratios may be used to obtain copolymers with different proportions of monomeric units.

Examples of compounds prepared by the above general processes and within the present invention have the following formulae:

FORMULA I In... L 1.

wherein n is at least 1 and m+n is greater than 10.

FORMULA II L L Li... L

wherein n is at least 1 and m+n is greater than 10.

3 FORMULA III 11 i Ncrn cm I L l I I IJm L 111 wherein n is at least 1 and m+n is greater than 10.

FORMULA VI ITI H l c a L It ILL I wherein n is at least 1 and rn+n is greater than 10.

FORMULA V wherein n is at least 1 and m+n is greater than 10.

FORMULA VI @Tl Li I L 1'1 L in wherein n is at least 1 and m+n is greater than 10.

FORMULA VII -C IlI I IJm L .J

wherein n is at least 1 and m+n is greater than 10.

FORMULA VIII l CC II I IJm L in wherein n is at least 1 and In-l-n is greater than 10.

FORMULA IX \N/ H NII L lit f1 In L I wherein n is at least 1 and m+n is greater than 10.

wherein n is at least 1 and m+n is greater than 10.

FORMULA XI @Tl L l l N-C II;

5;; L L I wherein n is at least 1 and 111+]: is greater than 10.

FORMULA XII ii? I L I wherein n is at least 1 and m+n is greater than 10.

FORMULA XIII a L In L I wherein n is at least I and m+n is greater than 10-.

FORMULA XIV I i N-CHQCII: C C

L l'i .J L in wherein n is at least 1 and m +n is greater than 10.

FORMULA XV CH3 N H L II I I Jm L in FORMULA XVI cc /s II I I Jm L In wherein n is at least 1 and m+n is greater than 10.

wherein n is at least 1 and 111+ is greater than 10.

FORMULA XVII L Hm wherein n is at least 1 and m+n is greater than 10.

FORMULA XVIII N H In L I H H wherein n is at least 1 and m+n is greater than 10.

FORMULA XIX C I llmL I.

wherein n is at least 1 and m+n is greater than 10.

FORMULA XX @TJ? L L ILL I.

wherein n is at least 1 and m+n is greater than 10.

FORMULA XXI L 0 W m p GC a 1'1 In wherein n is at least 1 and m+n is greater than 10.

FORMULA XXII wherein n is at least 1 and m-l-n is greater than 10.

FORMULA XXIII wherein n is at least 1 and m -l-n is greater than 10.

6 FORMULA XXIV For the preparation of photoconductive elements, it is advantageous for the compounds of the general formula to be soluble in organic solvents, such as tetrahydrofuran, or any other solvent in which compounds are at least partially soluble. Such solutions are applied to substrates suitable for electrophotography and the solvent is then removed. Mixtures of solvents can also be used. If the polymer is essentially insoluble, it can be melted and applied to the substrate while in a liquid state.

The substrate material, if one is desired, may be any which satisfy the requirements of electrophotography such as metal, glass, paper, or plastic. Unless it is to be used in a device employing dual corona discharge, such as US. 2,922,883, the substrate should preferably have a conductivity greater than l0 ohmcmf Applications of the solutions of the polymers of the general formula to the substrate is in the usual manner, such as spraying, doctor blade, meniscus, etc., followed by drying.

The polymers of the present invention absorb below the visible region of the electromagnetic spectrum, i.e. below 4000 A. Accordingly, their spectral sensitivity to the visible region may be improved by the addition of dyestulf sensitizers. In addition, activators or electron acceptors Which increase the photoconductivity of the polymers of the present invention may also be applied with the polymers in preparing the photoconductive element. When added in a layer with the polymer, the activator or electron acceptor may be in molar amounts up to and beyond equivalent molar amounts of each monomeric unit in the copolymer. The only limitation on the amount of the activator or electron acceptor in the polymer layer is the dark conductivity. That is, high concentration of some activators or electron acceptors increase the dark conductivity to an extent that the photoconductive element is not useful in electrophotography. Examples of dyestuif sensitizers and activators or electron acceptors useful with the polymers of the present invention are found in US. Patent 3,232,755.

One type of electrophotography process in which the polymers of the invention are useful is known as xerography. This process comprises the laying down of a uniform electrostatic charge on a photoconductive insulating layer comprising at least one of the polymers of the invention, exposing the electrostatic charge surface to a pattern of light to effect decay of the charge in the illuminated areas, and contacting the latent electrostatic image thus formed with colored electroscopic powder to render the image visible. Next, a copy sheet is brought into contact with the developed image and transferred thereto and fixed thereon. The residual powder remaining on the photoconductive insulating element is removed by cleaning and the element is then ready for the preparation of the next copy or the next cycle.

The general nature of the invention having been set forth, the following examples are now presented as to the specific preparation of polymers falling within the general formula and the specific preparation of these polymers into the photoconductive elements which are then used in electrophotographic processes. The specific details presented are for purposes of illustration.

Example I.-Five grame (0.035 mole) N-vinylindole and 4.1 g. (0.035 mole) indole were weighed into a heavy walled Pyrex polymerization tube. The mixture was de- Clrademark of Corning Glass Works.

gassed for several hours under vacuum, then sealed at mm. Hg. The tube was heated in an oil bath at 150 C. for 24 hours. The light yellow viscous product was dis solved in benzene and precipitated in a large volume of methyl alcohol. After drying for 14 hours at 60 C., 4.0 g. conversion) of a white powdery solid was obtained. The intrinsic viscosity of the polymer (Formula I) determined in benzene was 0.20. The melting point range was indefinite for the polymer turned brown at 250 C. and became black at 300 C.

The elemental analysis for (C H N calculated on basis of a /50 copolymer was as follows:

Calculated (percent): C, 83.04; H, 6.19; N, 10.76. Found (percent): C, 82.82, 83.01; H, 5.98, 6.10; N, 10.29, 10.34.

Example II.-Five grams of N-vinylindole (0.035 mole) and 4.58 g. (0.035 mole) freshly distilled N-methylindole were weighed into a heavy walled Pyrex polymerization tube. The tube contents were degassed and sealed as in Example I, then heated in an oil bath at 150 C. for 24 hours. The light yellow viscous product, obtained upon cooling, was dissolved in benzene and precipitated in 800 ml. methyl alcohol. The white precipitate was re-dissolved and re-precipitated, then dried at C. for 16 hours giving 6.2 gm. (64.5% conversion) White powdery polymer (Formula II). The intrinsic viscosity in benzene was 0.24 and the melting point range was 220 to 230 C. Elemental analysis for (C H N indicated a 50/ 50 copolymer:

Calculated (percent): C, 83.17; H, 6.61; N, 10.21. Found (percent): C, 83.19, 83.06; H, 6.42, 6.58; N, 10.17, 10.31.

Example III.N-vinylindole, 14.3 g. (0.01 mole) and 13.1 g. (0.1 mole) freshly distilled N-methylindole were added to ml. dry benzene in a 250 m1. 3-necked polymerization flask equipped with stirrer, thermometer, condenser and nitrogen gas inlet tube. The mixture was heated to 50 C. in an oil bath, then 0.33 g. (0.002 mole) azobisisobutyronitrile catalyst was added. The temperature of the mixture was raised to 70 C. and the polymerization continued for 24 hours under a nitrogen blanket. The mixture was cooled, poured into ml. methyl alcohol to give a heavy white precipitate. The precipitate was dissolved in benzene, filtered through a S-micron millepore filter into 800 ml. methyl alcohol. The precipitate was dried under vacuum 24 hours to give a 4.7 g. (17.5% conversion) white powdery solid. The polymer (Formula II) had an intrinsic viscosity of 0.06 and a softening range of 198216 C. Elemental analysis for (C H N indicated a 50/50 copolymer:

Calculated (percent): C, 83.17; H, 6.61; N, 10.21. Found (percent): C, 83.08, 83.17; H, 6,27, 6,29; N, 10.55, 10.67.

Example lV.Five grams (0.035 mole) pure N-vinylindole and 4.05 g. (0.035 mole) freshly distilled indene was mixed then transferred to a heavy walled Pyrex tube. The monomer mixture was degassed and sealed as in previous examples, then heated at C. for 96 hours and at 200 C. for an additional 20 hours. A clear viscous mass was obtained which was dissolved in hot tetrahydrofuran and precipitated as a white solid in 800 ml. methyl alcohol. Upon drying for 14 hours at 60 C. 3.9 g. (43% conversion) of a white powder polymer (Formula VI) was obtained which melted from to 173 C. and had an intrinsic viscosity in tetrahydrofuran of 0.05. The elemental analysis for (C H N) indicated 50/50 copolymer.

Calculated (percent): C, 88.34; H, 6.24; N, 542. Found (percent): C, 87.54, 87.72; H. 6.38, 6.50; N, 5.99, 6.05.

Example V.Five grams (0.035 mole) pure N-vinylindole and 5.1 g. (0.035 mole) coumarin were mixed intimately, then transferred to a heavy walled Pyrex glass polymerization tube. The mixture was degassed, sealed, then heated at 200 C. for 48 hours. The dark viscous mass obtained was twice dissolved in hot tetrahydrofuran and precipitated from cold methyl alcohol. A light tan solid polymer (Formula VIII) 4.5 g. (44.5% conversion) was obtained after drying 16 hours at 60 C.; M.P. 300 C. which had an intrinsic viscosity in tetrahydrofuran of 0.24. Elemental analysis for (C H NO Calculated (percent): C, 78.87; H, 5.23. Found (percent): C, 80.98, 8l.13; H, 5.78, 5.94.

Example VI.-N-vinylindolc, 10.9 g. (0.077 mole) and maleic anhydride, 7.5 g. (0.076 mole) were added to 285 g. of methylene chloride in a 500 m1. 3-necked polymerization flask equipped with stirrer, thermometer, condenser, and nitrogen gas inlet tube. The mixture was heated to 40 C. in an oil bath. Then 0.09 g. (3.8 10 mole) of benzoyl peroxide catalyst was added. The polymerization was continued for 22 hours under a nitrogen blanket. The mixture was cooled, poured into 1000 ml. of hexane to give 11.9 g. (68% conversion) of a red colored solid precipitate. The polymer (Formula VII) had a softening range of 146-153 C. The elemental analysis for calculated on the basis of a 85/15 copolymer was as follows:

Calculated (percent): C, 78.60; H, 5.70; N, 8.31. Found (percent): C, 78.46, 78.51; H, 5.83, 5.89; N, 7.90, 7.94.

Examples VlI-XII.-Photoconductive elements were prepared from the polymer of Examples I, II, IV, V, and VI by forming five separate solutions of 2,4,7-trinitro-9- flurorenone and each of the polymers, except for the polymer of Example VI, in a ratio of .7 mole of 2,4,7-trinitro-9-fluorenone to 1 monomeric unit of the copolymer using tetrahydrofuran as the solvent. The ratio of 2,4,7- trinitro-9-fiuorenone to the copolymer of Example VI was .62 to 1. The solutions were coated on five separate aluminum substrates with a doctor blade set on a 5 mil wet gap. After drying, the photoconductive layers had a thickness of 78 microns. The thus prepared photoconductive elements were tested for this photoconductive and electrophotographic properties on the electrometer described in US. Pat. S.N. 690,775, filed Dec. 15, 1967 and assigned to the same assignee. The results of tests are shown in the following table.

W; is the exposure time required to reach one-half of the original electrostatic surface potential applied to the photoconductive element.

While the invention has been shown and described with reference to preferred embodiments thereof, it will be appreciated by those skilled in the art that variations in form may be made therein without departing from the spirit and scope of the invention. For example, it will be understood that the compounds are not limited as photoconductors in the mode electrophotography known as xerography, but may be used in persistent electrophotographic methods such as that described in US. Pat. No. 2,845,348 or any other method where the photoconductor is exposed before charging. In addition, even though the greatest demand for the present invention is in conjunction with electrophotography, the compounds are also particularly well suited for other photoconductive applications.

What is claimed is:

1. An electrophotographic process comprising the steps of forming an electrostatic charge pattern on a photoconductive element comprising a photoconductive polymer having the following structural formula:

N I? R L 1% Him L 11 wherein R is selected from the group consisting of hydrogen and lower alkyl, wherein R is a fused ring radical, and wherein n is at least 1 and m+n is greater than 10.

2. The process of claim 1 wherein the polymer has the following structural formula:

R \N H 5-4: V L 111 1% mL 1..

wherein R is selected from the group consisting of hydrogen and lower alkyl, wherein X is selected from the group consisting of O, S, CH and NR wherein R is selected from the group consisting of hydrogen and lower alkyl, and wherein n is at least 1 and m+n is greater than 10.

3. The process of claim 1 wherein the polymer has 3 wherein R is selected from the group consisting of hydrogen and lower alkyl, and wherein n is greater than 10.

4. The process of claim 2 wherein the polymer has the following structural formula:

wherein n is at least 1 and m+n is greater than 10.

5. The process of claim 2 wherein the polymer has the following structural formula:

/N-OH3 L A A mL in wherein n is atleast 1 and m+n is greater than 10.

1O 6. The process of claim 1 wherein the polymer has the following structural formula:

H m L wherein n is at least 1 and m+n is greater than 10.

7. The process of claim 1 wherein the polymer has the following structural formula:

ill. L 1.

wherein n is at least 1 and m+n is greater than 10.

8. The process of claim 1 wherein the polymer has the following structural formula:

i L l ..L J.

wherein n is at least 1 and m+n is greater than 10.

9. The process of claim 1 wherein the polymer has the following structural formula:

l d-on T T L l. t mL 1.

wherein n is at least 1 and m+n is greater than 10.

10. The process of claim 1 wherein the polymer has the following structural formula:

Lg L a ELI... L J. wherein n is at least 1 and m+n is greater than 10.

References Cited UNITED STATES PATENTS 3,312,673 4/1967 Newett 26080.3

GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R. 

